Fix integer overflow when handling MPEG4 tx3g atom

When the sum of the 'size' and 'chunk_size' variables is larger than 2^32,
an integer overflow occurs. Using the result value to allocate memory
leads to an undersized buffer allocation and later a potentially
exploitable heap corruption condition. Ensure that integer overflow does
not occur.

Bug: 20923261
(cherry picked from commit 9f65a14d)

Change-Id: I81c8b074cb7f90d395d00a5c73b23f3f6c9341aa